Although various types of compressors are known, majority of them are of a type rotating a rotating body such as an impeller, screw rotor, scroll rotor, axial flow type vane and the like and compressing fluid accompanying rotation of the rotating body. Accordingly, in the compressor, a compressing space for accommodating the main section of the rotating body and compressing the fluid is formed, and a bearing section for supporting a shaft of the rotating body is formed. Also, in a gap between the compressing space and the bearing section and a gap between the compressing space and the atmospheric space, various seals are employed in order to prevent leakage of the compressed fluid from the compressing space and flowing-in of a lubricant (oil, grease and the like) and air and the like (particularly in a compressor and the like employing a so-called process gas as a fluid to be compressed) from the bearing section to the compressing space.
Particularly, with respect to the compressor that handles the gas of flammable and explosive gas such as hydrocarbon and the like, toxic gas, corrosive gas and the like as a fluid to be compressed, the constitution of the seal against the gas becomes important. These days, so-called dry gas seal draws attention as a dry seal that does not use oil at all as a material for sealing.
Generally speaking, the dry gas seal is constituted of a rotating ring rotating integrally with a shaft of a rotating body and stationary rings disposed in positions of the rotating ring opposite to vertical edge surfaces generally orthogonal to the shaft and fixed to the casing and the like via an elastic material. In the dry gas seal, in a state the rotating body stops, the stationary rings abut upon the rotating ring, seal surfaces are formed, and flowing out and the like of the gas to be compressed is prevented. Also, in most of the vertical edge surfaces of the rotating ring of the dry gas seal, namely the surfaces opposite to the stationary rings, spiral grooves are formed. Further, in a state the rotating body is rotating, the sealing gas flows in to the spiral grooves, dynamic pressure is formed, narrow gaps are formed between the rotating ring and the stationary rings, the seal surfaces for the sealing gas are formed there, and flowing out and the like of the gas to be compressed is also prevented.
In the Patent Literature 1, an example of a seal of a compressor by a dry gas seal is shown. As shown in FIG. 9, the seal of the Patent Literature 1 includes a dry gas seal section 104 and a barrier seal section 105 between a compressor casing 102 on the back of an impeller 101 and an impeller rotor shaft 103. The dry gas seal section 104 includes a stationary side dry gas seal body 106 fixed to the compressor casing 102 and a rotating side dry gas seal body 107 fixed to the impeller rotor shaft 103. Also, the dry gas seal section 104 is formed of a primary dry gas seal section 108 and a secondary dry gas seal section 109. In the primary dry gas seal section 108 and the secondary dry gas seal section 109, rotating rings 110A, 110B fixed to the rotary side dry gas seal body 107 and stationary rings 112A, 112B fixed to the stationary side dry gas seal body 106 via springs 111A, 111B are disposed respectively so as to be opposite to each other in the axial direction. On the surfaces of the rotating rings 110A, 110B opposite to the stationary rings 112A, 112B, spiral grooves not illustrated are formed. The barrier seal section 105 is fixed to the compressor casing 102 and is integrally connected to the stationary side dry gas seal body 106.
According to the seal disclosed in the Patent Literature 1, even when a thrust force under high pressure operation is large, the seal can withstand the thrust in terms of strength, and stability of the rotating body can be ensured.
Also, when liquid (drain and the like) infiltrates to the seal surface formed by the dry gas seal, dynamic pressure formed becomes unstable, and the gap is not formed stably between the rotating ring and the stationary rings. This phenomenon is considered to be caused because variation occurs in the force (floating force) generated between the rotating ring and the stationary rings because of coexistence of non-compressive liquid and compressive gas on the seal surfaces. Therefore, when the liquid infiltrates to the seal surfaces, the rotating ring and the stationary rings are brought into contact with each other, and the seal surfaces may possibly be damaged during rotation of the rotating body.